Patient monitor for determining microcirculation state

ABSTRACT

As placement of a physiological monitoring sensor is typically at a sensor site located at an extremity of the body, the state of microcirculation, such as whether vessels are blocked or open, can have a significant effect on the readings at the sensor site. It is therefore desirable to provide a patient monitor and/or physiological monitoring sensor capable of distinguishing the microcirculation state of blood vessels. In some embodiments, the patient monitor and/or sensor provide a warning and/or compensates a measurement based on the microcirculation state. In some embodiments, a microcirculation determination process implementable by the patient monitor and/or sensor is used to determine the state of microcirculation of the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/101,093, filed May 4, 2011, titled PATIENT MONITOR FOR DETERMININGMICROCIRCULATION STATE, which claims the benefit of priority under 35U.S.C. §119(e) of U.S. Provisional Application No. 61/332,155, filed May6, 2010, titled PATIENT MONITOR FOR DETERMINING MICROCIRCULATION STATE,the entire contents of each of which are hereby incorporated byreference herein in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates to medical sensors and specifically to amedical sensor and/or monitor for determining the circulation state inblood vessels.

BACKGROUND OF THE DISCLOSURE

Patient monitoring of various physiological parameters of a patient isimportant to a wide range of medical applications. Oximetry is one ofthe techniques that has developed to accomplish the monitoring of someof these physiological characteristics. It was developed to study and tomeasure, among other things, the oxygen status of blood. Pulseoximetry—a noninvasive, widely accepted form of oximetry—relies on asensor attached externally to a patient to output signals indicative ofvarious physiological parameters, such as a patient's constituentsand/or analytes, including for example a percent value for arterialoxygen saturation, carbon monoxide saturation, methemoglobin saturation,fractional saturations, total hematocrit, billirubins, perfusionquality, or the like. A pulse oximetry system generally includes apatient monitor, a communications medium such as a cable, and/or aphysiological sensor having light emitters and a detector, such as oneor more LEDs and a photodetector. The sensor is attached to a tissuesite, such as a finger, toe, ear lobe, nose, hand, foot, or other sitehaving pulsatile blood flow which can be penetrated by light from theemitters. The detector is responsive to the emitted light afterattenuation by pulsatile blood flowing in the tissue site. The detectoroutputs a detector signal to the monitor over the communication medium,which processes the signal to provide a numerical readout ofphysiological parameters such as oxygen saturation (SpO2) and/or pulserate.

High fidelity pulse oximeters capable of reading through motion inducednoise are disclosed in U.S. Pat. Nos. 7,096,054, 6,813,511, 6,792,300,6,770,028, 6,658,276, 6,157,850, 6,002,952 5,769,785, and 5,758,644,which are assigned to Masimo Corporation of Irvine, Calif. (“MasimoCorp.”) and are incorporated by reference herein. Advanced physiologicalmonitoring systems can incorporate pulse oximetry in addition toadvanced features for the calculation and display of other bloodparameters, such as carboxyhemoglobin (HbCO), methemoglobin (HbMet),total hemoglobin (Hbt), total Hematocrit (Hct), oxygen concentrations,glucose concentrations, blood pressure, electrocardiogram data,temperature, and/or respiratory rate as a few examples. Typically, thephysiological monitoring system provides a numerical readout of and/orwaveform of the measured parameter. Advanced physiological monitors andmultiple wavelength optical sensors capable of measuring parameters inaddition to SpO2, such as HbCO, HbMet and/or Hbt are described in atleast U.S. patent application Ser. No. 11/367,013, filed Mar. 1, 2006,titled Multiple Wavelength Sensor Emitters and U.S. patent applicationSer. No. 11/366,208, filed Mar. 1, 2006, titled NoninvasiveMulti-Parameter Patient Monitor, assigned to Masimo Laboratories, Inc.and incorporated by reference herein. Further, noninvasive bloodparameter monitors and optical sensors including Rainbow™ adhesive andreusable sensors and RAD-57™ and Radical-7™ monitors capable ofmeasuring SpO2, pulse rate, perfusion index (PI), signal quality (SiQ),pulse variability index (PVI), HbCO and/or HbMet, among otherparameters, are also commercially available from Masimo Corp.

During blood circulation, arteries carry blood away from the heart inhigh volume and under high pressure. Arteries branch off into smallerblood vessels, called arterioles. Arterioles are well innervated,surrounded by smooth muscle cells, and are about 10-100 μm in diameter.Arterioles carry the blood to the capillaries, which are the smallestblood vessels, which are not innervated, have no smooth muscle, and areabout 5-8 μm in diameter. Blood flows out of the capillaries into thevenules, which have little smooth muscle and are about 10-200 μm indiameter. The blood flows from venules into the veins, which carry bloodback to the heart.

Microcirculation generally refers to the vascular network lying betweenthe arterioles and the venules, including the capillaries, as well asthe flow of blood through this network. These small vessels can be foundin the vasculature which are embedded within organs and are responsiblefor the distribution of blood within tissues as opposed to largervessels in the macrocirculation which transport blood to and from theorgans. One of the functions of microcirculation is to deliver oxygenand other nutrients to tissue. Sometimes, microcirculation in thesesmall vessels can become blocked, interfering with the delivery ofoxygen to the tissue.

SUMMARY OF THE DISCLOSURE

As placement of a physiological monitoring sensor is typically at asensor site located at an extremity of the body, the state ofmicrocirculation, such as whether vessels are blocked or open, can havea significant effect on the readings at the sensor site. It is thereforedesirable to provide a patient monitor and/or physiological monitoringsensor capable of distinguishing the microcirculation state of bloodvessels. In some embodiments, the patient monitor and/or sensor providea warning and/or compensates a measurement based on the microcirculationstate. In some embodiments, a microcirculation determination processimplementable by the patient monitor and/or sensor is used to determinethe state of microcirculation of the patient.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the drawings, reference numbers may be re-used to indicatecorrespondence between referenced elements. The drawings are provided toillustrate embodiments of the disclosure described herein and not tolimit the scope thereof.

FIG. 1 illustrates a block diagram of a patient monitor, such as a pulseoximeter, and associated sensor;

FIG. 2 illustrates an example graph depicting the optical absorptioncharacteristic of normal blood and plasma;

FIGS. 3A and 3B illustrate graphs of oxygen saturation values for anormal microcirculation state data set;

FIGS. 4A and 4B illustrate graphs of oxygen saturation values foranother normal microcirculation state data set;

FIGS. 5A and 5B illustrate graphs of oxygen saturation values for ananomalous microcirculation state data set;

FIG. 6 illustrates a flow diagram for a process for determining thestate of microcirculation usable by a pulse oximeter; and

FIG. 7 illustrates a flow diagram for a process for determining thestate of microcirculation wherein multiple data points are collected.

DETAILED DESCRIPTION

FIG. 1 illustrates a block diagram of a patient monitor 100, such as apulse oximeter, and associated sensor 110. Generally, in the case of apulse oximeter, the sensor 110 has LED emitters 112, generally one at ared wavelength and one at an infrared wavelength, and a photodiodedetector 114. The sensor 110 is generally attached to an adult patient'sfinger or an infant patient's foot. For a finger, the sensor 110 isconfigured so that the emitters 112 project light through the fingernailand through the blood vessels and capillaries underneath. The LEDemitters 112 are activated by drive signals 122 from the pulse oximeter100. The detector 114 is positioned at the fingertip opposite thefingernail so as to detect the LED emitted light as it emerges from thefinger tissues. The photodiode generated signal 124 is relayed by acable to the pulse oximeter 100.

A pulse oximeter 100 determines oxygen saturation (SpO2) by computingthe differential absorption by arterial blood of the two wavelengthsemitted by the sensor 110. A typical pulse oximeter 100 contains asensor interface 120, one or more processors 130, such as a SpO2processor, an instrument manager 140, a display 150, an audibleindicator (tone generator) 160, and a keypad 170. The sensor interface120 provides LED drive current 122 which alternately activates thesensor's red and infrared LED emitters 112. The sensor interface 120also has input circuitry for amplification and filtering of the signal124 generated by the photodiode detector 114, which corresponds to thered and infrared light energy attenuated from transmission through thepatient tissue site. The SpO2 processor 130 calculates a ratio ofdetected red and infrared intensities, and an arterial oxygen saturationvalue is empirically determined based on that ratio. The instrumentmanager 140 provides hardware and software interfaces for managing thedisplay 150, audible indicator 160, and keypad 170. The display 150shows the computed oxygen saturation status, as described above.Similarly, other patient parameters including HbCO, HbMet, Hbt, Hct,oxygen concentrations, glucose concentrations, pulse rate, PI, SiQ,and/or PVI can be computed. The audible indicator 160 provides the pulsebeep as well as alarms indicating desaturation events. The keypad 170provides a user interface for such things as alarm thresholds, alarmenablement, and/or display options.

Computation of SpO2 relies on the differential light absorption ofoxygenated hemoglobin, HbO₂, and deoxygenated hemoglobin, Hb, todetermine their respective concentrations in the arterial blood.Specifically, pulse oximetry measurements are made at red (R) andinfrared (IR) wavelengths chosen such that deoxygenated hemoglobinabsorbs more red light than oxygenated hemoglobin, and, conversely,oxygenated hemoglobin absorbs more infrared light than deoxygenatedhemoglobin, for example 660 nm (R) and 905 nm (IR).

To distinguish between tissue absorption at the two wavelengths, the redand infrared emitters 112 are provided drive current 122 so that onlyone is emitting light at a given time. For example, the emitters 112 canbe cycled on and off alternately, in sequence, with each only active fora quarter cycle and with a quarter cycle separating the active times.This allows for separation of red and infrared signals and removal ofambient light levels by downstream signal processing. Because only asingle detector 114 is used, it responds to both the red and infraredemitted light and generates a time-division-multiplexed (“modulated”)output signal 124. This modulated signal 124 is coupled to the input ofthe sensor interface 120.

In addition to the differential absorption of hemoglobin derivatives,pulse oximetry relies on the pulsatile nature of arterial blood todifferentiate hemoglobin absorption from absorption of otherconstituents in the surrounding tissues. Light absorption betweensystole and diastole varies due to the blood volume change from theinflow and outflow of arterial blood at a peripheral tissue site. Thistissue site might also comprise skin, muscle, bone, venous blood, fat,pigment, and/or the like, each of which absorbs light. It is assumedthat the background absorption due to these surrounding tissues isinvariant and can be ignored. Thus, blood oxygen saturation measurementsare based upon a ratio of the time-varying or AC portion of the detectedred and infrared signals with respect to the time-invariant or DCportion: R/IR=(Red_(AC)/Red_(DC))/(IR_(AC)/IR_(DC)).

The desired SpO2 measurement is then computed from this ratio. Therelationship between R/IR and SpO2 can be determined by statisticalregression of experimental measurements obtained from human volunteersand calibrated measurements of oxygen saturation. In a pulse oximeterdevice, this empirical relationship can be stored as a “calibrationcurve” in a read-only memory (ROM) look-up table so that SpO2 can bedirectly read-out of the memory in response to input R/IR measurements.

The pulse oximeter 100 can also measure perfusion index, PI, which is anumerical value that indicates the strength of the IR signal returnedfrom a monitoring site and provides a relative assessment of the pulsestrength at the monitoring site. The perfusion index can be defined asfollows: PI=(IR_(max)−IR_(min))/IR_(DC), where IR_(max) is the maximumvalue, IR_(min) is the minimum value, and IR_(DC) is the average valueof the invariant portion. As the light absorption characteristic ofblood is typically “flatter” or less sensitive to oxygen saturationaround the infrared wavelength, the infrared signal from a sensor isinfluenced primarily by the amount of the blood at the monitoring site,not by the level of oxygenation in the blood. Accordingly, the perfusionindex, which is a numerical value that indicates the strength of the IRsignal returned from a monitoring site, provides a relative assessmentof the pulse strength at the monitoring site. PI values generally rangefrom 0.02% (very weak pulse strength) to 20% (very strong pulsestrength). In some embodiments, PI can be measured using otherwavelengths. For example, red, near red, near IR, as well as otherwavelengths can be used.

In an embodiment, the sensor 110 also includes a memory device 116. Thememory 116 can include any one or more of a wide variety of memorydevices known to an artisan from the disclosure herein, includingerasable programmable read only memory (EPROM), electrically erasableprogrammable read-only memory (EEPROM), flash memory, other non-volatilememory, a combination of the same or the like. The memory 116 caninclude read-only memory such as read-only memory (ROM), a read andwrite device such as a random-access memory (RAM), combinations of thesame, or the like. The remainder of the present disclosure will refer tosuch combination as simply EPROM for ease of disclosure; however, anartisan will recognize from the disclosure herein that the memory caninclude ROM, RAM, single wire memory, other types of memory,combinations of the same, or the like.

The memory device 116 can advantageously store some or all of a widevariety of data and information, including, for example, information onthe type or operation of the sensor, type of patient or body tissue,buyer or manufacturer information, sensor characteristics including thenumber of wavelengths capable of being emitted, emitter specifications,emitter drive requirements, demodulation data, calculation mode data,calibration data, software such as scripts, executable code, or thelike, sensor electronic elements, sensor life data indicating whethersome or all sensor components have expired and should be replaced,encryption information, monitor or algorithm upgrade instructions ordata, or the like. In an embodiment, the memory device can also includeoxygen saturation to perfusion index and R/IR ratio to perfusion indexratios and/or data.

In certain situations, pulse oximetry sensors may produce anomalousreadings, such as when a patient suffers from cyanosis. In a patientsuffering from cyanosis, blood cells are uncharacteristically low onoxygen, leading to oxygen deficiency and giving the patient's skin abluish-hue. One potential cause is that the patient's body produces toomuch hemoglobin, making the blood “thicker” or slower flowing, makingmicrocirculation vessels more prone to blockage. Thus, a “blocked”microcirculation state can indicate cyanosis.

A “blocked” microcirculation state can also indicate other medicalconditions, such as sepsis, systemic inflammatory response syndrome(SIRS), or septicemia. Sepsis is a potentially deadly medical conditionthat is characterized by a whole-body inflammatory state (called SIRS)and the presence of a known or suspected infection. The body may developthis inflammatory response by the immune system to microbes in theblood, urine, lungs, skin, or other tissue. Septicemia is a relatedmedical term referring to the presence of pathogenic organisms in thebloodstream, which can lead to sepsis. Sepsis can also be referred to asblood poisoning. During sepsis or SIRS, inflammation in the body cancause constriction in blood vessels, leading to low blood pressure orinsufficient blood flow.

During a “blocked” microcirculation state, blood cells can get blockedin the microcirculation vessels, such as the arterioles and capillaries.Blood cells can clump together or otherwise catch against the wall ofblood vessels, creating a blockage that prevents blood cells, includingred blood cells carrying hemoglobin, from passing through the blockage.However, plasma, which is composed of mostly water and in which theblood cells are suspended, is generally able to flow through passages inthe blockage. In some situations, some blood vessels at the monitoringsite may continue to have normal flow while some vessels are blocked.Thus, a “blocked” microcirculation state can indicate that somemicrocirculation vessels in an area are blocked and not necessarily allvessels in the area are blocked.

With the blockage preventing most or all the red blood cells frompassing a blood vessel, at most only a limited amount of hemoglobinpasses through a blocked blood vessel. In some situations, the bloodvessel may only be partially blocked, where some hemoglobin passesthrough but less than when the blood vessel is unblocked. Normally,blood is made up of about 40-50% of red blood cells, of which about 95%is hemoglobin. Plasma, which is about 95% water, normally constitutesabout 55% of the blood's volume.

Accordingly, a pulse oximeter placed on a tissue site experiencingblockage in microcirculation vessels may detect mostly plasma passingthrough with no or only a small percentage of red blood cells, at leastat part of the monitoring site. The resulting change in the normalcomposition of blood can cause anomalous readings in the pulse oximetrymonitor. As plasma has generally different absorption characteristicsfor red and infrared wavelengths than normal blood, pulse oximetryreadings may become skewed. Red_(AC) and/or IR_(AC) can be affected,causing measured R/IR ratio to change. For example, if Red_(AC) rises orIR_(AC) drops, the R/IR ratio increases. Alternatively, if Red_(Ac)drops or IR_(AC) rises, the R/IR ratio decreases. Thus, the value ofR/IR can change due to a change in the light absorption of blood even ifthe underlying oxygen saturation of the blood remains the same.

However, by comparing oxygen saturation and PI for normalmicrocirculation to the oxygen saturation and PI for blockedmicrocirculation, such as by calculating and comparing ratios, themonitor can determine the existence of an abnormal situation. Typically,SpO2 is mostly independent of PI, with SpO2 varying minimally as PIincreases. However, SpO2 varying by more than normal as PI increases canindicate an anomalous microcirculation state, such as a blockage. In oneembodiment, by analyzing the measured ratios, the pulse oximeter 100 candetermine the microcirculation state, such as whether a blocked vesselexists in the microcirculation vessels.

FIG. 2 illustrates an example graph depicting the optical absorptioncharacteristic of normal blood and plasma. The graph depicts samplingwavelengths at 660 nm 220 and at 905 nm 225. As illustrated, IRabsorption for plasma at a frequency of 905 nm is on a “steeper” sectionof the curve compared to the “flatter” section of the curve for normalblood. This can imply that readings for IR for plasma would be moresensitive to changes in the absorption quality of the blood. Incontrast, the IR measurement for normal blood, for example at 905 nm, isusually insensitive to a change in oxygenation of normal blood, but moreaffected by change in the amount of blood. As illustrated in the graph,plasma can have a “flatter” section in its absorption curve at adifferent wavelength, for example at 970 nm 230.

FIGS. 3A and 3B illustrate graphs of oxygen saturation values for anormal microcirculation state data set. FIG. 3A has an y-axis 305corresponding to the measured ratio, R/IR, and a x-axis 310corresponding to perfusion index, PI. FIG. 3B has a y-axis 320corresponding to measured oxygen saturation, and an x-axis 325corresponding to perfusion index, PI. FIGS. 3A and 3B represent multipledata points with a best fit line 315, 330 indicating the trend of thedata points. Each data point represents a measurement. As illustrated,the best fit line for FIG. 3A trends slightly downward and the best fitline for FIG. 3B trends slightly upwards. However, there is generallyonly a small change in the y-axis for the best fit line as PI increases,with the change in FIG. 3A around 0.1 and the change in FIG. 3B around4.

FIGS. 4A and 4B illustrate graphs of oxygen saturation values foranother normal microcirculation state data set. FIG. 4A has a y-axis 405corresponding to the measured ratio, R/IR, and an x-axis 410corresponding to perfusion index, PI. FIG. 4B has a y-axis 420corresponding to measured oxygen saturation, and an x-axis 425corresponding to perfusion index, PI. FIGS. 4A and 4B represent multipledata points with a best fit line 415, 430 indicating the trend of thedata points. Each data point represents a measurement. As illustrated,the best fit line for FIG. 4A trends slightly upwards and the best fitline for FIG. 4B trends slightly downwards. However, there is generallyonly a small change in the y-axis for the best fit line as PI increases,with the change in FIG. 4A around 0.1 and the change in FIG. 4B around3.

FIGS. 5A and 5B illustrate graphs of oxygen saturation values for ananomalous microcirculation state data set. FIG. 5A has a y-axis 505corresponding to the measured ratio, R/IR, and an x-axis 510corresponding to perfusion index, PI. FIG. 5B has a y-axis 520corresponding to measured oxygen saturation, and an x-axis 525corresponding to perfusion index, PI. FIGS. 5A and 5B represent multipledata points with a best fit line 515, 530 indicating the trend of thedata points. Each data point represents a measurement. As illustrated,the best fit line for FIG. 5A trends significantly upwards on the y-axisby around 0.3 and the best fit line for FIG. 5B trends significantlydownwards on the y-axis by around 13 as PI increases.

In comparison to FIGS. 3A and 4A, FIG. 5A shows a high R/IR ratio forlow values of PI that becomes a high R/IR ratio as PI increases. Incomparison to FIGS. 3B and 4B, FIG. 5B shows a high reading for lowvalues of PI that becomes a low reading as PI increases. Differencesbetween the graphs can be explained by the microcirculation state inFIGS. 5A and 5B being different from the microcirculation state in FIGS.3A-4B. For example, FIGS. 5A and 5B can represent a “blocked” orpartially blocked microcirculation state where the blood passing throughthe sensor includes mostly plasma. As discussed above, this can skewR/IR and the measured oxygen saturation derived from R/IR.

FIG. 6 illustrates a flow diagram for a process 600 for determining thestate of microcirculation usable by a pulse oximeter. Microcirculationstate can be determined by comparison with microcirculation data storedon a patient monitor, such as the pulse oximeter 100 of FIG. 1. Theprocess 600 can be implemented by embodiments of the sensor 110 and/orpatient monitor 100 of FIG. 1 or other suitable device.

While in conventional pulse oximetry, measurements are generally takenpulse-by-pulse and averaged over pulses, microcirculation measurementscan be measured using only a single pulse or a portion of a singlepulse. This can be done, for example, at the minimum and/or maximumblood flow of a pulse. Microcirculation measurements can also bedetermined over multiple pulses. In some embodiments, microcirculationmeasurements are taken during a portion of the normal measurement timeused by a physiological sensor to take a measurement of a parameter,thereby allowing detection of aberrant parameter measurements using themicrocirculation measurements. For example, while a pulse oxymeter ismeasuring SpO2 over several pulses, microcirculation measurements can betaken per pulse and a warning given if an irregular microcirculationstate is detected, thereby notifying a user of a possible aberration inthe current SpO2 reading.

At block 610, oxygen saturation is measured at a tissue monitoring site.In one embodiment, oxygen saturation is determined using a pulseoximeter sensor.

At block 620, perfusion index or pulse strength is measured. In oneembodiment, the perfusion index is determined using the same sensor usedto measure oxygen saturation so that readings are taken at the samemonitoring site.

At block 630, a ratio of oxygen saturation to perfusion index isdetermined. Oxygen saturation can be a SpO2 value based on the measuredR/IR ratio looked-up against a calibration curve. Alternatively, theratio can be perfusion index to oxygen saturation. In other embodiments,the measured R/IR ratio can be used directly instead of SpO2.

In some embodiments, multiple readings of perfusion index and oxygensaturation can be taken and averaged together before determining theratio in order to account for outliers. The multiple readings can befiltered before averaging. For example, readings can first be filteredbased on closeness of PI values before the readings are averagedtogether.

At block 640, the determined ratio in block 630 is compared to storedmicrocirculation data. The stored data can be data sets formicrocirculation states. In some embodiments, a ratio, a curve, a line,table, data points, or formula can be stored that corresponds to a dataset. The measured perfusion index and oxygen saturation can then becompared to the stored data. In some embodiments, multiple readings aretaken and a best fit line or curve is generated and compared to a storedbest fit line or curve. In some embodiments, readings are collected atvarious PI values in order to generate a trend line.

At block 650, the microcirculation state is determined from comparisonof the stored microcirculation data. For example, if the determinedratio is similar to a stored ratio corresponding to a data set forunblocked microcirculation, the microcirculation state is determined tobe unblocked. Other data sets for other microcirculation states, such asblocked and/or partially blocked can also be stored. Where multiple datasets are stored, the state can be determined by selecting the statecorresponding to the stored ratio closest to the measured ratio.

At block 660, the monitor can optionally generate an alarm and/ordisplay the microcirculation state. For example, an alarm signal can begenerated by the monitor to indicate that the readings may be anomalous,such as when a blocked or partially blocked microcirculation state isdetected. The alarm can be a visual indicator (e.g., icon, message orimage) and/or an audio indicator. In an embodiment, the alarm canindicate the detection of cyanosis, sepsis, SIRS or other medicalcondition based at least partly on the determined microcirculationstate. In some situations, no action is taken, such as when readings aredetermined to be normal or non-threatening.

At block 670, the monitor can optionally compensate for themicrocirculation state in order to improve accuracy of the readings.After the microcirculation state returns to normal, the compensationprocess can be ended.

In one embodiment, an offset can be added to the measured parametervalue, such as SpO2. The offset can be calculated based on data sets formicrocirculation state. Different microcirculation states can havedifferent offsets. For example, if a “blocked” microcirculation stateproduces high readings for low PI values, a negative offset can be used.However, if a “blocked” state produces a low value for high PI values,then a positive offset can be used. In one embodiment, a varying offsetcan be used depending on the value of PI.

In one embodiment, a different wavelength emitter can be used tocompensate for a microcirculation state. For example, rather than usinga regular infrared emitter, typically 905 nm, an emitter with adifferent infrared wavelength, such as 970 nm can be used. In oneembodiment, the different wavelength is selected such that thewavelength is at a “flat” section of the light absorption curve forplasma, that is, where the light absorption is not much affected bychanges in oxygen saturation. In one embodiment, the selected wavelengthwith regards to plasma mimics the properties of the regular wavelengthwith regards to normal flowing blood. In some embodiments, a differentwavelength red emitter can be used instead of the regular red wavelengthemitter.

In some embodiments, the pulse oximeter sensor used to measure oxygensaturation and PI can be provided with an additional emitter at adifferent wavelength than the existing emitters. When a certainmicrocirculation state is detected, such as a “blocked” state, theadditional emitter can be used. For example, a pulse oximetry sensor canbe equipped with LED's capable of emitting at 660 nm, 905 nm, and at 970nm wavelengths. Under normal operation, the 660 nm and 905 nm emittersare active. However, upon detecting a blocked microcirculation state,the 905 nm emitter can be deactivated and the 970 nm emitter activatedin its place. In some embodiments, a variable wavelength emitter can beused rather than separate emitters. In some embodiments, the additionalemitter can be a red wavelength emitter.

FIG. 7 illustrates a flow diagram for a process 700 for determining thestate of microcirculation wherein multiple data points are collected.The process 700 can be implemented by embodiments of the sensor 110and/or patient monitor 100 of FIG. 1 or other suitable device.

At block 710 and block 720, oxygen saturation and perfusion index aremeasured. At block 725, measured values are stored in memory. Eachpaired measurement forms a data point.

At block 730, the number of stored data points is checked to determineif sufficient data has been collected to determine the microcirculationstate. Data can be sufficient if a set number of data points have beencollected, a set amount of time has passed, and/or a spectrum of datapoints have been collected, such as for differing values of PI.

At block 740, the stored measured data is compared with storedmicrocirculation data. Typically, the microcirculation data ispre-stored on the pulse oximeter before use, as opposed to collectedduring use. A comparison can involve generating a curve or line from themeasured data, calculating a rate of change for the stored data,generating a trend line for the measured data or the like and comparingwith the stored microcirculation data.

At block 750, the microcirculation state is determined from comparisonof the stored microcirculation data. For example, if the measured datais similar to microcirculation data corresponding to a data set forunblocked microcirculation, the microcirculation state is determined tobe unblocked. Other data sets for other microcirculation states, such asfor blocked and/or partially blocked can also be stored. Where multipledata sets are stored, the state can be determined by selecting the statecorresponding to the stored ratio closest to the measured ratio.

Blocks 760 and 770 are similar to steps 660 and 670 described in FIG. 6.

As will be apparent from the above description, the R/IR ratiocorresponds to oxygen saturation or SpO2 and can be used in place ofoxygen saturation or SpO2 for the above comparisons, and vice versa.

While the above systems and methods have been described in terms ofoxygen saturation and PI, other physiological parameters can be measuredin place of or in addition to oxygen saturation and/or perfusion indexand used to determine microcirculation state. For example, perfusionindex is an indication of amplitude and/or signal strength and otherparameters or measurements indicating amplitude and/or signal strengthcan be used. In some embodiments, one or more different sensors can beused in place of or in addition to a pulse oximeter sensor.

Conditional language used herein, such as, among others, “can,” “could,”“might,” “may,” “e.g.,” and the like, unless specifically statedotherwise, or otherwise understood within the context as used, isgenerally intended to convey that certain embodiments include, whileother embodiments do not include, certain features, elements and/orstates. Thus, such conditional language is not generally intended toimply that features, elements and/or states are in any way required forone or more embodiments or that one or more embodiments necessarilyinclude logic for deciding, with or without author input or prompting,whether these features, elements and/or states are included or are to beperformed in any particular embodiment. The terms “comprising,”“including,” “having,” and the like are synonymous and are usedinclusively, in an open-ended fashion, and do not exclude additionalelements, features, acts, operations, and so forth. Also, the term “or”is used in its inclusive sense (and not in its exclusive sense) so thatwhen used, for example, to connect a list of elements, the term “or”means one, some, or all of the elements in the list.

Various systems and processes for determining microcirculation statehave been disclosed in detail in connection with various embodiments.These embodiments are disclosed by way of examples only and are not tolimit the scope of the claims that follow. Indeed, the novel methods andsystems described herein can be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe methods and systems described herein can be made without departingfrom the spirit of the inventions disclosed herein. The claims and theirequivalents are intended to cover such forms or modifications as wouldfall within the scope and spirit of certain of the inventions disclosedherein. One of ordinary skill in the art will appreciate the manyvariations, modifications and combinations. For example, the variousembodiments of the microcirculation determination process can be usedwith other oxygen saturation sensors and with both disposable andreusable sensors. In some embodiments, the determination process can beapplied to other blood vessels to detect a blockage, even in vessels notinvolved in microcirculation.

Furthermore, in certain embodiments, the systems and methods describedherein can advantageously be implemented using computer software,hardware, firmware, or any combination of software, hardware, andfirmware. In one embodiment, the system includes a number of softwaremodules that comprise computer executable code for performing thefunctions described herein. In certain embodiments, thecomputer-executable code is executed on one or more general purposecomputers or processors. However, a skilled artisan will appreciate, inlight of this disclosure, that any module that can be implemented usingsoftware can also be implemented using a different combination ofhardware, software or firmware. For example, such a module can beimplemented completely in hardware using a combination of integratedcircuits. Alternatively or additionally, such a module can beimplemented completely or partially using specialized computers orprocessors designed to perform the particular functions described hereinrather than by general purpose computers or processors.

Moreover, certain embodiments of the invention are described withreference to methods, apparatus (systems) and computer program productsthat can be implemented by computer program instructions. These computerprogram instructions can be provided to a processor of a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructions,which execute via the processor of the computer or other programmabledata processing apparatus, create means for implementing the actsspecified herein to transform data from a first state to a second state.

Each of the processes, methods, and algorithms described in thepreceding sections may be embodied in, and fully or partially automatedby, code modules executed by one or more computers or computerprocessors. The code modules may be stored on any type of non-transitorycomputer-readable medium or computer storage device, such as harddrives, solid state memory, optical disc, and/or the like. The processesand algorithms may be implemented partially or wholly inapplication-specific circuitry. The results of the disclosed processesand process steps may be stored, persistently or otherwise, in any typeof non-transitory computer storage such as, e.g., volatile ornon-volatile storage.

The various features and processes described above may be usedindependently of one another, or may be combined in various ways. Allpossible combinations and subcombinations are intended to fall withinthe scope of this disclosure. In addition, certain method or processblocks may be omitted in some implementations. The methods and processesdescribed herein are also not limited to any particular sequence, andthe blocks or states relating thereto can be performed in othersequences that are appropriate. For example, described blocks or statesmay be performed in an order other than that specifically disclosed, ormultiple blocks or states may be combined in a single block or state.The example blocks or states may be performed in serial, in parallel, orin some other manner. Blocks or states may be added to or removed fromthe disclosed example embodiments. The example systems and componentsdescribed herein may be configured differently than described. Forexample, elements may be added to, removed from, or rearranged comparedto the disclosed example embodiments.

While certain example embodiments have been described, these embodimentshave been presented by way of example only, and are not intended tolimit the scope of the inventions disclosed herein. Thus, nothing in theforegoing description is intended to imply that any particular feature,characteristic, step, module, or block is necessary or indispensable.Indeed, the novel methods and systems described herein may be embodiedin a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the methods and systemsdescribed herein may be made without departing from the spirit of theinventions disclosed herein. The accompanying claims and theirequivalents are intended to cover such forms or modifications as wouldfall within the scope and spirit of certain of the inventions disclosedherein.

What is claimed is:
 1. A patient monitor configured to determine amicrocirculation state, the patient monitor comprising: a sensorinterface configured to receive a signal from a physiological sensor,the signal indicative of first and second measurement valuescorresponding to respective first and second physiological parameters,wherein the physiological sensor is configured to transmit light of atleast one wavelength into tissue of a patient and to detect the lightafter it has been attenuated by the tissue, and wherein the secondphysiological parameter comprises a measure of perfusion; and one ormore processors configured to: determine a relationship between thefirst and second physiological parameters; and compare the determinedrelationship between the first and second physiological parameters withstored lookup data indicative of a microcirculation state of bloodvessels to determine if a blocked or partially blocked microcirculationstate exists in at least a portion of blood vessels in the tissue;wherein the one or more processors are further configured to generate adrive signal to configure the physiological sensor to measure a lightabsorption characteristic of the tissue at a different wavelength inresponse to the determination of the microcirculation state.
 2. Thepatient monitor of claim 1, wherein the first physiological parametercomprises oxygen saturation.
 3. The patient monitor of claim 2, whereinthe measure of perfusion comprises a perfusion index.
 4. The patientmonitor of claim 3, wherein the physiological sensor comprises a pulseoximeter sensor.
 5. The patient monitor of claim 1 further comprising analarm configured to activate when a blocked or partially blockedmicrocirculation state is determined to exist.
 6. The patient monitor ofclaim 5, wherein the alarm is configured to activate when the blocked orpartially blocked microcirculation state is indicative of cyanosis. 7.The patient monitor of claim 5, wherein the alarm is configured toactivate when the blocked or partially blocked microcirculation state isindicative of sepsis.
 8. A method of non-invasively determining amicrocirculation state, the method comprising: obtaining, at a tissuesite by a non-invasive physiological sensor, first and secondmeasurement values corresponding to respective first and secondphysiological parameters, the second physiological parameter comprisinga measure of perfusion; determining a relationship between the first andsecond physiological parameters; comparing the determined relationshipbetween the first and second physiological parameters with stored lookupdata indicative of a microcirculation state of blood vessels todetermine if a blocked or partially blocked microcirculation stateexists in at least a portion of blood vessels at the tissue site; andapplying an offset to at least one of the first physiological parameteror the second physiological parameter to compensate for a blocked orpartially blocked microcirculation state.
 9. The method of claim 8,wherein the first physiological parameter comprises oxygen saturation.10. The method of claim 9, wherein the measure of perfusion comprises aperfusion index.
 11. The method of claim 10, wherein the non-invasivephysiological sensor comprises a pulse oximeter sensor.
 12. The methodof claim 8, wherein determining a relationship between the first andsecond physiological parameters comprises generating a ratio of thefirst measurement value to the second measurement value.
 13. The methodof claim 8, wherein determining a relationship between the first andsecond physiological parameters corresponds to generating a trend linebased on the stored data points of the first and second physiologicalparameters.
 14. The method of claim 8, wherein the stored lookup datacomprises at least one lookup curve.
 15. The method of claim 8 furthercomprising generating an alarm signal when a blocked or partiallyblocked microcirculation state is determined.
 16. A patient monitorconfigured to determine a microcirculation state, the patient monitorcomprising: a sensor interface configured to receive a signal from aphysiological sensor, the physiological sensor configured to transmitlight of at least one wavelength into living tissue of a patient and todetect the light after it has been attenuated by the tissue; and one ormore processors configured to determine a microcirculation state of thetissue based on at least a first physiological parameter derived from ananalysis of detected attenuated light, and at least a measurement ofperfusion; wherein the one or more processors are further configured togenerate a drive signal to configure the physiological sensor to measurethe light absorption characteristic of the tissue monitoring site at adifferent wavelength in response to the determination of themicrocirculation state.
 17. The patient monitor of claim 16, wherein themicrocirculation state comprises one of an unblocked, a blocked, or apartially blocked state.
 18. The patient monitor of claim 17 furthercomprising an alarm configured to activate when a blocked or partiallyblocked microcirculation state is detected.
 19. The patient monitor ofclaim 16, wherein the physiological sensor comprises a pulse oximetersensor.
 20. The patient monitor of claim 16 further comprising an alarmconfigured to activate when cyanosis is detected based at least partlyon the microcirculation state of the tissue.
 21. The patient monitor ofclaim 16 further comprising an alarm configured to activate when sepsisis detected based at least partly on the microcirculation state of thetissue.